A furnace and method for producing an article of relatively pure, fused oxide.
Relatively pure, metal oxides are produced by thermal decomposition of precursors and deposition of the resulting oxides. The precursor may take the form of a vapor, or may be carried by a vapor. It may be decomposed by either flame hydrolysis or pyrolysis.
One such process is production of fused silica by hydrolysis or pyrolysis of silicon tetrachloride. Early patents disclosing such processes for producing silica are U.S. Pat. Nos. 2,239,551 (Nordberg) and 2,272,342 (Hyde). A commercial application of flame hydrolysis involves forming and depositing particles of fused silica to form large bodies (boules). Such boules may be used individually, or may be finished and integrated together into large optical bodies, such as telescope mirrors. In this procedure, SiCl4 is hydrolyzed, and the hydrolyzed vapor is passed into a flame to form molten particles of fused silica. The particles are continuously deposited on a bait, or in a crucible, known as a cup, to form a boule.
Other fused, metal oxide materials are known, of course, but fused silica has achieved major commercial significance. The present disclosure, therefore, refers, generically, to xe2x80x9cfused metal oxides,xe2x80x9d but is primarily concerned with fused silica. However, a modification, designed to achieve a near-zero coefficient of thermal expansion, is composed of about 93% SiO2 and 7% TiO2. The term, xe2x80x9cfused silica,xe2x80x9d then includes such compositions where silica is the dominant oxide, and where the physical properties are closely related.
A serious drawback, in such a process for producing a metal oxide from the corresponding chloride, has been the need to dispose of the HCl byproduct in an environmentally safe manner. Accordingly, it has been proposed, in U.S. Pat. No. 5,043,002 (Dobbins et al.), to employ a halide-free, silicon-containing compound as a substitute for SiCl4. In particular, the patent proposes using a polymethylsiloxane, such as octamethylcyclotetrasiloxane, to provide the vaporous reactant for the hydrolysis or pyrolysis process.
In order to introduce a substitute precursor, it is, of course, critically necessary to avoid any significant change in the properties of the fused silica product. Unfortunately, the substitution proposed by the Dobbins et al. patent did lead to significant property changes. One such change was a reduction in UV transmission properties. Another was development of fluorescence in the glass that increased when the glass was exposed to short wavelength radiation.
Studies revealed that a major factor in the transmission loss was sodium ion content in the glass. U.S. Pat. Nos. 5,332,702 and 5,395,413 (Sempolinski et al.) describe remedial measures taken to reduce the sodium ion content. In particular, it was found necessary to use dispersants, binders and water relatively free of sodium ions in producing zircon refractory components for the furnace. Essentially, these measures provided a purer zircon refractory. This refractory was used in constructing the furnace in which the fused silica was deposited to form a boule.
An improved product was obtained by adopting the practices prescribed in the Sempolinski et al. patents. However, attempts to use the fused silica in certain applications made it apparent that further improvements were necessary to meet the critical requirements of these applications. One such application is lenses designed for transmission of very short, UV-wavelength radiation from an excimer type laser. This laser emits radiation at about 193 nm and 248 nm wavelengths.
It was found that lenses produced from available fused silica did not provide acceptable transmission of the short wavelength radiation, and exhibited an undesirable fluorescence. Both of these conditions tend to become worse with service time. The loss of transmission, or darkening of the glass, is commonly referred to as UV absorption damage.
Improvement in the zircon refractory, as disclosed in the Sempolinski et al. patents, alleviated the affect of sodium ion contamination in a fused silica article. However, it was then found that, in addition to sodium, other contaminants also exist in the furnace refractory in damaging amounts. These include the alkaline earth metals, and transition metals, such as iron, titanium and lead, aluminum, phosphorous and sulfur.
These metal contaminants have varying degrees of volatility at temperatures in excess of 1650xc2x0 C., the temperature at which fused silica is deposited. Thus, they may be present in the furnace atmosphere, and become entrapped in the fused silica as it is deposited. The presence of these contaminating metals in a fused silica lens results in a reduction of the transmittance capability of the glass. It also results in development of an undesirable fluorescence in the glass. These deficiencies continue to further develop as the lens is subjected to short wavelength, UV radiation in service.
There are inherent variations in the metal impurity levels in a refractory material, as well as varying degrees of metal volatility. This makes it difficult to control glass quality in a collection furnace for fused silica, or even to obtain acceptable glass frequently. The problem becomes particularly acute when a polysiloxane is used as a precursor material for the fused silica. As explained in the Sempolinski et al. patents, the self-cleansing action of the HCl by-product from a SiCl4 decomposition is lost with the siloxane precursor.
Contaminating metals can be present in the raw materials employed in production of furnace refractories. The metals may also be entrained during sintering of the refractory, or during any subsequent operations, such as sawing or grinding. Zircon is a relatively clean refractory, particularly when prepared as described in the Sempolinski et al. patents. However, the superior transmission properties required for such demanding uses as microlithography applications require control of all metal contaminants at a level below 100 parts per billion (ppb).
Comparative studies have revealed that the required degree of contaminating metal control in a collection furnace can be achieved by constructing the furnace from refractory materials containing less than 300 parts per million (ppm) of the contaminating metals.
In an effort to achieve this degree of purity, two procedures have been developed. One involves constructing at least a portion of the collection furnace, in particular the crown, from refractory materials that have been exposed to a reactive, halogen-containing gas. The gas reacts with, and thereby cleanses, the refractory of contaminating metals. This process is described in greater detail, and claimed, in U.S. Pat. No. 6,174,509 issued Jan. 16, 2001 (Pavlik, Jr. et al.).
In general, the process entails exposing the refractory brick to a reactive, halogen-containing gas at a temperature of 700-1500xc2x0 C. Conveniently, this can coincide with firing of the refractory brick. In particular, it is convenient that the treatment occur as the fired refractory is being cooled, for example, at a temperature of about 1200xc2x0 C.
The other procedure, commonly referred to as xe2x80x9ccarbochlorination,xe2x80x9d also involves treatment at an elevated temperature in the range of 1000-1500xc2x0 C. The treatment may be carried out in a graphite vessel having a bed of loose carbon. The vessel also has means for evacuation, and for controlling input and exhaust of treatment gases, particularly chlorine.
Both procedures are effective to reduce the presence of contaminating metals, particularly sodium and iron, in a metal oxide, refractory body, such as zircon. However, both procedures involve the use of environmentally unfriendly materials in a gas form at greatly elevated temperatures.
It would, therefore, be desirable to provide a procedure that lends itself more readily to control. The present invention is the result of research aimed at meeting this need.
The invention resides in part in a method of producing a fused oxide body by decomposing a precursor compound of the oxide in a flame to form molten oxide particles that are collected to form a fused oxide body in a furnace constructed of a refractory material, the improvement in the method comprising treating the refractory material with a strong acid in liquid form to react with, and thereby remove, contaminants from at least the surface of the refractory material.
The invention further resides in a refractory furnace for collecting fused oxide particles to form a solid oxide body, at least a portion of the furnace being constructed with a refractory brick from which contaminants have been removed by treatment of the brick in an acid bath.